Reduction of power consumption is required for a heater including a ceramic substrate formed mainly of alumina, and a heat-generating resistor provided on the ceramic substrate. In order to reduce the power consumption of the heater, desirably, generation of heat is suppressed to a minimum possible level in a lead portion of the heat-generating resistor, the resistor including the lead portion and a heat-generating portion. Therefore, desirably, the ratio of the resistance of the heat-generating portion to the sum of the resistances of the heat-generating portion and the lead portion (hereinafter the ratio may be referred to as “heating-portion resistance ratio”) is increased. For example, Japanese Patent Application Laid-Open (kokai) No. 2000-58237 (“Patent Document 1”) discloses a heater including a heat-generating resistor formed of tungsten, wherein the heating-portion resistance ratio is 75% or more.
In general, when the heating-portion resistance ratio of the heat-generating resistor increases, a heat-generating portion of the resistor has increased temperature elevation rate, whereby a ceramic substrate provided on the resistor is rapidly heated. However, regarding the heater described in Patent Document 1, no particular attention is paid to durability against peeling of the heat-generating resistor from the ceramic substrate, which could be caused by repeated rapid heating and cooling, since the difference in thermal expansion coefficient between tungsten and alumina is relatively small. In contrast, in the case of a heater including a heat-generating resistor containing a noble metal (e.g., platinum, palladium, or rhodium) as a main component, and a ceramic substrate containing alumina as a main component, the difference in thermal expansion coefficient between the noble metal and alumina is large. Thus, when heating-portion resistance ratio increases, the heat-generating resistor is more likely to be peeled from the ceramic substrate due to thermal shock. Therefore, demand has arisen for a technique regarding a heater which realizes both reduction of power consumption and improvement of durability against thermal shock, the heater including a ceramic substrate containing alumina as a main component, and a heat-generating resistor containing a noble metal as a main component.